1. Field of the Invention
The present invention relates to an image forming apparatus such as copiers, printers, facsimiles and direct digital platemakers, and a process cartridge used therein.
2. Description of the Related Art
In electrophotographic image forming apparatuses, residual toner remaining on the surface of a photoreceptor even after a toner image thereon is transferred onto a recording material or an intermediate transfer medium is removed therefrom using a cleaner.
Strip-shaped cleaning blades made of an elastic material such as polyurethane rubbers are typically used for such a cleaner because of having advantages such that the cleaner has simplified structure and good cleanability. Among such cleaning blades, a cleaning blade in which one end thereof is supported by a supporter, and an edge line of the other end is contacted with a surface of a photoreceptor to block and scrape off residual toner on the photoreceptor, thereby removing the residual toner from the surface of the photoreceptor.
In attempting to meet a recent need of forming high quality images, there are image forming apparatuses using spherical toner (hereinafter referred to as polymerization toner), which has a relatively small particle diameter and which is prepared by a method such as polymerization methods. Since such polymerization toner has such an advantage as to have higher transfer efficiency than pulverization toner, which has been conventionally used, the polymerization toner can meet the need. However, polymerization toner has such a drawback as not to be easily removed from a photoreceptor by a cleaning blade. This is because such polymerization toner has a spherical form and a small particle diameter, and easily passes through a small gap between the tip of a cleaning blade and the surface of a photoreceptor.
In attempting to prevent polymerization toner from passing through a gap between a cleaning blade and a photoreceptor, it is necessary to increase the pressure to the cleaning blade contacted with the surface of the photoreceptor to enhance the cleanability of the cleaning blade.
However, as disclosed in Japanese published unexamined application No. JP-2010-191378-A, when the contact pressure of the cleaning blade is increased, the friction between the cleaning blade and the photoreceptor is increased, and thereby the tip of the cleaning blade is pulled by the photoreceptor in the moving direction of the photoreceptor. Specifically, as illustrated in FIG. 8(a), a cleaning blade 62 is pulled by the surface of an photoreceptor 123 in a moving direction (indicated by an arrow) of the photoreceptor due to increase of friction between the blade and the photoreceptor, thereby causing a problem (hereinafter referred to as everted-tip problem) in that an edge line 62c of a tip 62a of the blade 62 is everted. In this regard, the thus everted tip has a restoring force, and therefore the tip tends to vibrate, resulting in generation of fluttering sounds. In addition, when the cleaning operation is continued while the edge line 62c of the cleaning blade 62 is everted, a portion of the tip 62a of the cleaning blade 62, which is apart from the edge line 62c by few micrometers, is abraded as illustrated in FIG. 8(b). When the cleaning blade 62 is further used for the cleaning operation, the portion of the tip 62a of the blade 62 is further abraded, resulting in lack of the edge line 62c of the blade 62 as illustrated in FIG. 8(c). The cleaning blade 62 having no edge line cannot remove residual toner from the surface of the photoreceptor 123, thereby forming an abnormal image in which background thereof is soiled with residual toner.
Japanese published unexamined application No. JP-2010-191378-A discloses a cleaning blade formed of a low-friction elastic blade, the surface of which is impregnated with at least one of an isocyanate compound, a fluorine compound and a silicone compound; and a surface layer covering an edge line of the elastic blade, formed of a UV curing resin harder than the elastic blade.
The cleaning blade having an edge line a surface layer harder than the elastic blade is formed on can prevent the edge line from deforming in a travel direction of a photoreceptor. Further, even when the surface layer is worn out and an edge line of the elastic blade is exposed, the impregnated part thereof contacts the photoreceptor and a frictional force between the elastic blade and the photoreceptor is reduced to prevent the exposed part from deforming. This prevents the edge line from being everted and increases abrasion resistance of the cleaning blade to prevent poor cleaning.
Japanese Patent No. JP-3602898-B1 (Japanese published unexamined application No. JP-H09-127846-A) discloses a cover layer made of a resin, which is harder than a rubber and has a pencil hardness of from B to 6H, is formed at least on the edge line of the tip of a cleaning blade. Japanese published unexamined application No. JP-2004-233818-A discloses a cleaning blade formed of an elastic blade impregnated with an ultraviolet crosslinkable material including a silicone so as to be swelled and surface layer harder than the elastic blade on at least a part of the cleaning blade contacting a photoreceptor, which is formed when the elastic blade is exposed to ultraviolet rays.
However, even when the above-mentioned cleaning blades are used, occurrence of the above-mentioned problems is hardly prevented if images having a high image area proportion (such as image having large solid images) are continuously produced (i.e., if the amount of residual toner on a photoreceptor to be removed by the cleaning blade is large). The reason is considered to be as follows.
Specifically, since the blade has a cover layer on the tip thereof or includes a crosslinked material in a surface portion thereof in the longitudinal direction thereof, the elastic property of the rubber of the blade tends to deteriorate. When the elastic property of the blade is deteriorated, the blade cannot be satisfactorily contacted with the surface of a photoreceptor (i.e., the pressure of the blade to a photoreceptor varies) if the photoreceptor is eccentric or the surface thereof is waved. In addition, when images having high image area proportions are continuously produced and a large amount of residual toner is present on the surface of the photoreceptor, the large amount of toner is collected at the tip of the blade by being blocked by the blade. In this case, the residual toner at the tip of the blade tends to pass through a relatively large gap formed between a portion of the blade and the surface of the photoreceptor, which are contacted with each other at a relatively low pressure due to eccentricity of the photoreceptor or waving of the surface thereof, resulting in occurrence of the above-mentioned abnormal image problem.
In the cleaning blade including an elastic blade formed a urethane rubber impregnated with an isocyanate compound and a surface layer harder than the elastic blade, disclosed in Japanese published unexamined application No. JP-2010-191378-A, the isocyanate compound chemically reacts with the urethane rubber, resulting in high crosslink density of the impregnated part of the elastic blade. Such an elastic blade deteriorates in elasticity and followability to eccentric variation of a photoreceptor, resulting in poor cleanability.
In the cleaning blade including an elastic blade and a surface layer formed of a resin having a pencil hardness of from B to 6H, disclosed in Japanese Patent No. JP-3602898-B1 (Japanese published unexamined application No. JP-H09-127846-A), the surface layer does not have enough abrasion resistance and is likely to early disappear due to friction with a photoreceptor. Then, when the surface layer thickness is made thicker, the elastic blade deteriorates in elasticity and followability to eccentric variation of a photoreceptor, resulting in poor cleanability.
In the cleaning blade formed of an elastic blade impregnated with an ultraviolet crosslinkable material including a silicone so as to be swelled and surface layer harder than the elastic blade on at least a part of the cleaning blade contacting a photoreceptor, which is formed when the elastic blade is exposed to ultraviolet rays, disclosed in Japanese published unexamined application No. JP-2004-233818-A, a large amount of the ultraviolet crosslinkable material needs to be impregnated to form a surface layer having sufficient hardness. However, when a large amount of the ultraviolet crosslinkable material is impregnated, the impregnated part is formed too hard and deep, resulting in deterioration of elasticity of the elastic blade. Therefore, the edge line of the cleaning blade deteriorates in followability to the surface of a photoreceptor, resulting in poor cleanability.
In addition, when a cleaning blade having an edge line harder than the elastic blade is used, the surface of a photoreceptor is abraded earlier than when only the elastic blade is used, resulting in deterioration of image quality such as background fouling.
Japanese published unexamined application No. JP-2010-191378-A discloses an image forming apparatus using a photoreceptor the cleaning blade having an edge line harder than the elastic blade contacts, including a surface layer formed of a crosslinkable charge transport material. However, the image forming apparatus just includes a combination of a cleaning blade and a photoreceptor each having improved mechanical durability. Hard layers contact each other with friction, abrasion of the cleaning blade or the photoreceptor is accelerated as time passes, resulting in poor cleanability.
Further, when the image forming apparatus using having a hard edge line continues to produce images on which a toner is eccentrically consumed in an axial direction of a photoreceptor, the surface of the photoreceptor is abraded in proportion to the toner consumption eccentricity, i.e., the photoreceptor is eccentrically abraded. Even a photoreceptor including a surface layer formed of a crosslinkable charge transport material is eccentrically abraded.
Because of these reasons, a need exist for an image forming apparatus using a cleaning blade including an elastic blade and an edge line harder than the elastic blade, in which the edge line has good followability to a photoreceptor to maintain good cleanability, and preventing the photoreceptor from being eccentrically abraded.